System and Method for a Collaborative Service Set

ABSTRACT

In one embodiment, a collaborative service set (CSS) includes a controller access point (AP) configured to be associated with a first plurality of stations and a first member AP, where the first member AP is associated with a second plurality of stations, where the controller AP is configured to coordinate transmissions between the first member AP and the second plurality of stations with transmissions between the controller AP and the first plurality of stations, where the controller AP and the first member AP are configured to transmit messages simultaneously.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/693,527 filed on Aug. 27, 2012 and entitled “System and Methodfor Collaborative Service Set for Wireless Local Area Networks,” andU.S. Provisional Application Ser. No. 61/693,651 filed on Aug. 27, 2012and entitled “System and Method for Channel Sounding in Wireless LocalArea Networks,” which applications are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a system and method for wirelesscommunications, and, in particular, to a system and method for acollaborative service set (CSS).

BACKGROUND

Wireless local area networks (WLANs) link wireless devices. Under theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standards, a physical layer standard for implementing WLANcommunications, access points (APs) communicate with associated stations(STAs). APs are base stations for wireless networks that transmit andreceive wireless communications from stations. An AP and its associatedstations may be configured in a basic service set (BSS), where thestations are associated with the AP in the BSS. As the demand forwireless throughput increases, APs are more densely populated. It isdesirable for densely located APs to coordinate transmissions.

Interference alignment (IA) is a multi-device transmission method inwhich interfering transmitters pre-code their signals in unwanted users'receive space, enabling these receivers to successfully acquire signalsintended for them.

SUMMARY

An embodiment collaborative service set (CSS) includes a controlleraccess point (AP) configured to be associated with a first plurality ofstations and a first member AP, where the first member AP is associatedwith a second plurality of stations, where the controller AP isconfigured to coordinate transmissions between the first member AP andthe second plurality of stations with transmissions between thecontroller AP and the first plurality of stations, where the controllerAP and the first member AP are configured to transmit messagessimultaneously.

An embodiment method of channel sounding in a collaborative service set(CSS) includes broadcasting, by a first access point (AP), a first nulldata packet announcement (NDPA) including a short identification number(ID) of a first station and broadcasting, by the first AP, a first nulldata packet (NDP) after transmitting the first NDPA. The method alsoincludes receiving, by the first AP from a second AP, a second NDPA andreceiving, by the first AP from the second AP, a second NDP afterreceiving the second NDPA. Additionally, the method includestransmitting, by the first AP to the first station, a first pollingframe after transmitting the first NDP and receiving the second NDP andreceiving, by the first AP from the first station, a first feedbackreport after transmitting the first polling frame.

An embodiment method of channel sounding in a collaborative service set(CSS) includes receiving, by a station from a first access point (AP), afirst null data packet announcement (NDPA) including a first simplifiedshort identification number (ID) of the station and receiving, by thestation from the first AP, a first null data packet (NDP) afterreceiving the first NDPA. The method also includes receiving, by thestation from a second AP, a second NDPA after receiving the first NDPand receiving, by the station from the second AP, a second NDP afterreceiving the second NDPA. Additionally, the method includes estimatinga first channel between the station and the first AP in accordance withthe first NDP to produce a first estimated channel and receiving a firstpolling frame, by the station from the first AP. Also, the methodincludes broadcasting, by the station, a first feedback report inaccordance with the first estimated channel after receiving the firstpolling frame.

An embodiment method of determining an oscillator frequency offsetbetween a first access point (AP) and a second AP includes receiving, bythe second AP from the first AP, a first packet including a firstpreamble and receiving, by the second AP from the first AP, a secondpacket including a second preamble after receiving the first packet.Additionally, the method includes determining an oscillator frequencyoffset between the first AP and the second AP in accordance with thefirst preamble and the second preamble.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates an embodiment basic service set (BSS);

FIG. 2 illustrates an embodiment extended service set (ESS);

FIG. 3 illustrates an embodiment overlapping basic service set (OBSS);

FIG. 4 illustrates an embodiment collaborative service set (CSS);

FIG. 5 illustrates an embodiment protocol for channel sounding;

FIG. 6 illustrates another embodiment CSS;

FIG. 7 illustrates another embodiment protocol for channel sounding;

FIG. 8 illustrates a flowchart for an embodiment method of channelsounding performed by an access point;

FIG. 9 illustrates a flowchart for an embodiment method of channelsounding performed by a station;

FIG. 10 illustrates a diagram with different oscillator frequencies fortwo access points;

FIG. 11 illustrates an embodiment protocol for channel sounding whilecompensating for an oscillator frequency offset;

FIG. 12 illustrates a flowchart for an embodiment method of channelsounding while compensating for an oscillator frequency offset;

FIG. 13 illustrates a graph of signal to noise ratio (SNR) versus biterror rate (BER) for joint multi-user beamforming with an oscillatorfrequency error and with perfect synchronization;

FIG. 14 illustrates a graph of SNR versus BER for access pointcollaborative beamforming with an oscillator frequency error and withperfect synchronization;

FIG. 15 illustrates synchronization of APs using short training fields(STFs) and long training fields (LTFs);

FIG. 16 illustrates a graph of SNR versus mean squared error (MSE) forcarrier frequency offset (CFO) estimation; and

FIG. 17 illustrates a block diagram of an embodiment of ageneral-purpose computer system.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

FIG. 1 illustrates basic service set (BSS) 100 for communicating data ina wireless local area network (WLAN). BSS 100 comprises access point(AP) 102 having a coverage area 104 and a plurality of stations (STAs)106. Five stations are pictured, but many more may be present. AP 102may be any component capable of providing wireless access by, interalia, establishing uplink and/or downlink connections with stations 106,such as base stations, enhanced base stations (eNBs), picocells,femtocells, and other wirelessly enabled devices. Stations 106 may beany components capable of establishing a wireless connection with AP102, such as cell phones, smart phones, tablets, laptop computers,sensors, etc.

FIG. 2 illustrates extended service set (ESS) 110, which contains BSSs120, 122, 124, and 126 that are connected by distribution system (DS)136. DS 136 enables communications between stations. In one example, DS136 is wired Ethernet. In another example, DS 136 is a wirelessconnection. Each BSS contains an AP (112, 114, 116, and 118,respectively) with a coverage area (121, 123, 125, and 127,respectively) and a plurality of stations associated with the AP (128,130, 132, and 134, respectively). ESS 110 is deployed and managed by asingle entity. All BSSs use the same service set identity (SSID), andthe station IDs may not be unique. Stations may freely roam between APsin ESS 110. ESS 110 has an extended range compared to BSS 100. BSSs 120,122, 124, and 126 are independent, and compete for the wireless medium.

FIG. 3 illustrates overlapping basic service set (OBSS) 140, anothersituation where APs interact. OBSS 140 contains BSSs 150, 152, 154, and156. The BSSs contain an AP (142, 144, 146, and 148, respectively) witha coverage area (151, 153, 155, and 157, respectively), with a pluralityof stations (158, 161, 163, and 165, respectively) coupled to the AP. Inan example, an OBSS occurs in neighboring buildings or in an apartmentbuilding where each tenant deploys a separate AP. OBSS 140 is deployedand managed by independent entities (APs 142, 144, 146, and 148). TheBSSs have separate SSIDs, and the station IDs may not be unique. To movefrom one AP to another, a station disassociates from an AP andassociates with another AP.

Table 1 below compares characteristics of ESSs and OBSSs. Both ESSs andOBSSs contain multiple APs that operate independently from each other.Each AP has its own associated stations and its own timing andsynchronization functions. There is little to no coordination orexchange of information between APs in an ESS or an OBSS. In both ESS110 and OBSS 140, multiple APs are implicitly coordinated using thecarrier sense multiple access/collision avoidance (CSMA/CA) and theclear channel assessment (CCA) function. An AP or a station that sensesthe wireless medium is busy defers its transmission to avoid collisions,so only a single device is transmitting at a time. Thus, at most onedevice is transmitting at a time within range of each other.

TABLE 1 ESS OBSS Deployed and managed by a Deployed and managed byindependent single entity entities DS exists No DS Single SSID MultipleSSIDs STA ID may not be unique STA ID may not be unique STAs can freelyroam between STAs cannot move from one AP to the APs in the same ESSother without de-associating and then associating

FIG. 4 illustrates collaborative service set (CSS) 160, whichfacilitates the collaboration among multiple APs (162, 164, 166, and168) located nearby to improve throughput. The APs are associated with aplurality of stations (176, 178, 180, and 182, respectively). Coveragearea 170 is an extended cloud for CSS 160. In CSS 160, multiple devicesmay transmit simultaneously. Signaling channel 174 couples APs 162, 164,166, and 168 and pluralities of stations 176, 178, 180, and 182. CSS 160enables stations to move freely among different APs without losingconnectivity. Also, CSS 160 contains distribution system 172.

In CSS 160, one AP is designated as a controller AP, and supervises theinter-AP coordination. The other APs are member APs. In one example, thecontroller AP is one of the member APs, for example AP 162 is thecontroller AP. In another example, the controller AP is a separateentity that makes use of the services of one or more member APs todispatch control messages. In this example, the controller APcommunicates with the member APs using distribution system 172. In bothexamples, the controller AP controls and supervises the collection ofchannel information and simultaneous transmissions from the member APs.The member APs are registered with the controller AP.

The stations in CSS 160 are given unique short identification numbers(IDs) that are shorter than their media access control (MAC) addresses.The short ID may be based on an association ID (AID). Alternatively, theshort ID is a unique ID based on another assignment. When an AID isused, the uniqueness of the AID may be facilitated by dividing the AIDspace between the APs. For example, if the AID space includes 2000 IDsand there are three APs, the first AP is assigned AIDs 1-667 the secondAP is assigned AIDs 668-1334, and the third AP is assigned AIDs1335-2000. In another example, each AP requests a short ID from thecontroller AP for its associated stations and conveys the assignedvalue. In one example, the short IDs are assigned by the controller AP.In another example, the short unique IDs are assigned in a distributedfashion by the member APs for their associated stations. The short IDsare available for the controller AP and the member APs.

The APs and associated stations have access to signaling channel 174.Signaling channel 174 may be used to exchange control informationbetween the APs and the stations.

When member APs associate with the CSS, they are given a rank or order.Distribution system 172 may be used for registration. Alternatively,registration is performed by exchanging management frames using awireless media. In one example, the member APs are within transmissionrange of each another. In another example, some member APs are not intransmission range of some other member APs. When not all member APs arein range, the controller AP may select a subset of member APs that arewithin range of each other, and assign a leader AP of the member APs toperform the coordination function. Alternatively, the leader AP is theAP that currently has access to the wireless media. The leader AP hastemporary control of the CSS operation for the duration of thetransmission opportunity. Then, the leader AP cedes control at the endof the transmission opportunity or when no frames are available totransmit. In an example, the throughput is increased by enablingmultiple APs in a CSS to transmit at a time. Thus, the data rate maygrow linearly.

Stations may associate with CSS 160 similarly to how stations associatewith an ESS. A station may associate with any member AP that is withinrange. A station does not need to be within the range of all member APs.The controller AP may propagate member AP information to the stations.

Table 2 below compares features of a CSS, an ESS, and an OBSS.

TABLE 2 CSS ESS OBSS Deployed and Deployed and Deployed and managed by amanaged by a managed by single entity single entity independent entitiesDS exists DS exists No DS Single SSID Single SSID Multiple SSIDs STA canfreely STA can freely STA cannot move roam between APs roam between APsfrom one AP to in the same CSS in the same ESS the other withoutde-associating and then associating Existence of a No controller Nocontroller controller Unique STA ID STA ID may not STA ID may not beunique be unique Existence of No signaling No signaling signalingchannel channel channel A leader Single device Single device coordinatestransmission at transmission at multiple-device any point in the anypoint in the transmissions time time

In an example, a simplified CSS is formed, where all stations associatewith the controller AP and all member APs and stations are within rangeof the controller AP. In this arrangement, the uniqueness of theassigned AIDs is assured without the need to exchange informationbetween the member APs. The controller AP may assign stations to themember APs to distribute the load. The range of a simplified CSS islimited to the range of a single BSS. Thus, a simplified CSS increasesthroughput without increasing range.

In an embodiment, CSS 160 is backward compatible with other WLANarchitectures and with legacy devices. In a dense WLAN deploymentenvironment, a CSS may coexist with other WLAN deployments. APs andstations of a CSS may use CCA and CSMA/CA to gain access to the wirelessmedium. Upon gaining access to the wireless medium, a CSS station mayinvite other stations belonging to the CSS to act simultaneously toimprove the throughput during the duration of the granted opportunity.

FIG. 5 illustrates protocol 380 for collecting channel feedback in a BSSwith AP 382 associated with stations 384, 386, and 388. Initially, AP382 broadcasts a null data packet announcement (NDPA). Then, after ashort interframe spacing (SIFS), AP 382 broadcasts a null data packet(NDP) containing only a physical (PHY) layer header, without a MACheader. In response, after a SIFS, station 384 transmits a compressedbeamforming report. Then, after a SIFS, AP 382 transmits a beamformingreport poll. In response, after a SIFS, station 386 transmits acompressed beamforming report. Next, after a SIFS, AP 382 transmitsanother beamforming report poll. Finally, after a SIFS, station 388responds with a compressed beamforming report. The stations follow inorder from the announcement frames.

FIG. 6 illustrates CSS 190 for channel sounding with interferencealignment. CSS 190 contains AP 192 associated with stations 191 and 193,and AP 194 associated with stations 195, 197, and 199. Distributionsystem 196 couples AP 192 and AP 194. To implement interferencealignment in a wireless network, knowledge of the channels betweeninterfering devices is collected and distributed among these devices.Ten channels between APs 192 and 194 and stations 191, 193, 195, 197,and 199 may be estimated. CSS 190 facilitates cooperation betweendevices participating in interference alignment. In an example, achannel sounding protocol for downlink interference alignment andchannel estimation for interference alignment is based on CSS 190.Channel information is collected to facilitate multiple devicetransmissions using interference alignment.

In an example, explicit channel sounding, which may be used forbeamforming, uses null data packets (NDPs) for channel sounding. An NDPis a data packet containing only the PHY layer header. The NDP istransmitted from an AP to stations when channel information isrequested. The NDP is preceded by an NDP announcement (NDPA) thatincludes the station IDs of the target stations. The AP then polls thetarget stations to request feedback reports containing the estimatedchannel information.

In an example, a channel sounding protocol is based on the cooperationamong a group of APs in a CSS for collecting and transmitting channelinformation. The APs select sets of target stations to participate ininterference alignment. In a simplified CSS, the controller AP mayperform the selection for the entire CSS without involving the othermember APs.

Each station that is a member of the CSS has a unique ID that is notdependent on which AP that station is associated with. The unique IDsare available to the controller AP and the member APs to enable memberAPs to unambiguously correlate channel feedback to the correspondingstation.

FIG. 7 illustrates protocol 200 for channel sounding in a CSS. Protocol200 includes communications between AP 192, AP 194, station 191, station193, station 195, station 197, and station 199. Initially, a leader APis selected. The leader AP may be the controller AP, the AP thatcurrently has access to the wireless media, or an AP that is assigned bythe controller AP. In protocol 200, AP 192 is selected as the leader AP.

Initially, AP 192 signals the start of the channel sounding process. TheAP 192 broadcasts an NDPA to signal the identity of the stations forwhich feedback is desired. The NDPA contains the short IDs of the targetstations. In protocol 200, the target stations are stations 191, 193,195, 197, and 199. The NDPA may include a field in its PHY header tosignal the start of the process. Alternatively, AP 192 broadcasts aframe to signal the start of the channel sounding process to theparticipating APs and stations. This frame may include the identities ofthe participating APs and stations. After broadcasting the NDPA, AP 192broadcasts an NDP with sufficient long training fields (LTFs) for thestations to perform channel estimation.

After a short interframe spacing, the non-leader APs broadcast an NDPA,followed by an NDP. One AP broadcasts an NDPA followed by an NDP. Then,the next AP broadcasts an NDPA followed by an NDP. This continues untilall participating APs have broadcast an NDPA and an NDP. In one example,the order of the APs is based on their rank. In another example, theleader AP assigns a temporary order to participating APs. Allparticipating APs transmit their NDPAs and NDPs before the stationstransmit their feedback reports. In protocol 200, there are two APs, butthere may be many more APs.

After the APs have transmitted the NDPAs and NDPs, the stations estimatethe channel between themselves and the APs. The NDP is composed of asmany LTFs as the number of transmissions. The NDP enables the estimationof channels regardless of the multiple-input multiple-output (MIMO) sizeand configuration. The stations then broadcast their feedback reports orbeamforming (BF) reports. This is initiated when an AP transmits apolling frame to a target AP. The target station responds bybroadcasting its feedback report. The AP polls all of its targetstations. The end of the polling phase for a particular AP may besignaled by including a bit in the polling frame that indicates the endof the polling frame. Then, the next AP begins its polling of targetstations. The APs proceed in rank order to poll the target stations. Thetarget stations respond by broadcasting their feedback reports. Thiscontinues until the stations broadcast the channels between themselvesand the participating APs. Broadcasting the feedback reports enables thechannel information to reach all of the participating APs. To reduce theprotocol overhead, the feedback report includes estimated channelinformation for all the participating APs. The format of the feedbackreports may be a compressed V matrix, a precoder index, or anotherformat.

By the end of protocol 200, each participating AP has the estimatedchannel information for each of the participating stations. Simultaneousmulti-AP transmissions may follow based on a client selection criterion.

Protocol 200 may be referred to as explicit feedback, because thechannel estimation process starts by explicitly transmitting NDPA andNDP frames from the APs to the stations. In another embodiment, channelestimation is performed without explicitly transmitting the NDPA and NDPframes. All transmitted frames in the WLAN environment include in theirPHY headers a set of long training fields (LTFs) that can be used forchannel estimation. The stations then continuously estimate the channelbetween themselves and the transmitting APs. Channel information maylater be requested for interference alignment.

FIG. 8 illustrates flowchart 230 for a method of channel soundingperformed by an AP. Initially, in step 231, the sequence of the APs isdetermined. The leader AP is the first AP. The leader AP may be thecontroller AP, the AP that currently has access to the wireless medium,or a member AP that is assigned to be the leader AP by the controllerAP. The remaining member APs have an order, which may be predetermined,or may be assigned by the controller AP. In a simplified CSS, only thecontroller AP communicates with the stations.

When the AP is the leader AP, it proceeds directly to step 232. When theAP is not the leader AP, it proceeds to step 239 to receive the NDPAfrom the leader AP. The NDPA includes the short IDs for the targetstations of the broadcasting APs.

Then, in step 241, the AP receives the NDP from the leader AP. The NDPcontains only a PHY header.

In step 243, the AP determines if there are additional APs to transmitNDPAs and NDPs before that AP. When there are additional APs, the APproceeds to step 239 to receive the NDPA from the next AP. When thereare no additional APs, the AP proceeds to step 232 to transmit its NDPA.

When it is the turn of the AP, it transmits an NDPA in step 232. TheNDPA contains station IDs of the stations targeted by the AP.

After a SIFS, the AP transmits an NDP to the target stations in step234. Then, the AP waits until all participating APs have transmitted anNDPA and an NDP frame.

The AP receives an NDPA from another member AP in step 233 when thereare APs later in the order.

Next, the AP receives an NDP from that member AP in step 235.

In step 237, the AP determines if there are additional APs to receive anNDPA and NDP from. When there are more APs to transmit an NDPA and NDP,the AP proceeds to step 233 to receive the next NDPA. When there are nomore APs, the AP proceeds to step 238.

The APs then poll the stations in the same sequence they transmittedtheir NDPA and NDP frames. In step 238, the AP transmits a polling frameto a particular station when it is that AP's turn to poll. When it isthe turn of another AP to poll, the AP receives a polling frame fromanother AP.

In response, in step 240, the AP receives a feedback report in step 240.The AP receives feedback reports from the target station it polls, andfrom the target stations that other APs poll.

Finally, in step 242, the AP determines if there are more feedbackreports. When there are no more feedback reports, the method ends instep 244. This occurs when all the APs have polled the target stations.However, when there are more feedback reports, the AP goes to step 238,to repeat the optional polling step, then to step 240 to receive thenext feedback report.

FIG. 9 illustrates flowchart 250 for a method of channel soundingperformed by a station. Initially, in step 252, the station receives anNDPA from the leader AP. The station determines whether its station IDis contained in the NDPA.

When the station ID is contained in the NDPA, the station receives anNDP containing the short IDs of the target stations from the leader APin step 254. When the short ID is not contained in the NDPA, the stationskips step 254.

Next, in step 256, if there are additional NDPAs and NDPs, the stationproceeds to step 252 to receive an NDPA from the next AP. When there areno more NDPAs, the station proceeds to step 258. In a simplified CSS,only the controller AP broadcasts NDPAs and NDPs.

In step 258, the station estimates the channel between itself and thefirst AP it received an NDP from.

Next, in step 260, the station receives a polling frame from the APs.

In response, in step 262, the station broadcasts a feedback report tothe APs with the channel information between the station and the leaderAP.

In step 264, the station determines if there are additional APs that itreceived NDPs from. If there are no additional APs, the method ends instep 268. If there are additional APs, the station proceeds to step 260to receive a polling frame from the next AP. In a simplified CSS, onlythe controller AP polls the stations.

In a CSS, the APs may be synchronized. FIG. 10 illustrates CSS 290containing two APs, AP 292 and AP 294, and two stations, station 296 andstation 298. A controller AP, for example AP 292, indicates a member AP,for example AP 294, for concurrent transmission. This is performed afterstations have been associated with the CSS, APs have been selected and achannel sounding protocol has been performed, such as protocol 200.There is a latency between APs, for example between AP 292 and AP 294,that may be measured in advance. Then, the latency can be compensatedfor by delaying the transmission between the APs. However, the phaseoffset caused by a different oscillator frequency for the APs isproblematic to compensate for.

The channel between AP 292, AP 294, station 296, and station 298 isgiven by:

${{H(t)} = \begin{bmatrix}{h_{00}^{{j{({\omega_{0} - \omega_{R_{0}}})}}t}} & {h_{01}^{{j{({\omega_{1} - \omega_{R_{0}}})}}t}} \\{h_{10}^{{j{({\omega_{0} - \omega_{R_{1}}})}}t}} & {h_{11}^{{j{({\omega_{1} - \omega_{R_{1}}})}}t}}\end{bmatrix}},$

where

2πf₀=ω₀,2πf₁=ω₁,2πf_(R) ₀ =ω_(R) ₀ ,2πf_(R) ₁ =ω_(R) ₁ .

The time offset in the time domain channel H(t) may be estimated bydecomposing it into three matrices, R(t)HT(t), where:

${{R(t)} = \begin{bmatrix}^{{- {j\omega}_{R_{0}}}t} & 0 \\0 & ^{{- {j\omega}_{R_{1}}}t}\end{bmatrix}},{H = \begin{bmatrix}h_{00} & h_{01} \\h_{10} & h_{11}\end{bmatrix}},{and}$ ${T(t)} = {\begin{bmatrix}^{{- {j\omega}_{0}}t} & 0 \\0 & ^{{- {j\omega}_{1}}t}\end{bmatrix}.}$

The channel does not change by multiplying by:

1=e^(jω) ⁰ ^(t)e^(−jω) ⁰ ^(t).

Thus,

H(t) = ^(jω₀)R(t)HT(t)^(−jω₀t), and ${H(t)} = {\begin{bmatrix}^{{- {j{({\omega_{0} - \omega_{R_{0}}})}}}t} & 0 \\0 & ^{{- {j{({\omega_{0} - \omega_{R_{1}}})}}}t}\end{bmatrix}{{H\begin{bmatrix}1 & 0 \\0 & ^{{j{({\omega_{1} - \omega_{0}})}}t}\end{bmatrix}}.}}$

The leftmost matrix indicates the frequency offset between AP 292 andstations 296 and 298, while the rightmost matrix indicates the frequencyoffset between AP 292 and AP 294. The offset between the APs and thestations is determined based on short training fields (STFs) and longtraining fields (LTFs).

FIG. 11 illustrates protocol 310 for channel sounding while compensatingfor an oscillator frequency offset caused by the different oscillatorfrequencies for different APs. Initially, the leader AP, AP 312,broadcasts an NDPA, followed by an NDP. When the NDPA and NDP arebroadcast from AP 312, AP 314 receives those packets, along with thestations. Using the STF and LTF in the preamble of NDPA and NDP, theoscillator frequency offset (OFO) between the APs may be estimated.Before AP 314 broadcasts an NDPA and NDP, some processing time is usedto estimate the OFO. When all the NDPAs and NDPs have been transmitted,the stations (316, 318, 320, and 322) receive polling requests from theAPs in sequence, and respond with a feedback report. As pictured, onlyAP 312 polls the stations. Station 316 is polled first, followed bystation 318, station 320, and station 322, respectively.

Alternatively, the OFO is estimated with a special pulse train sent fromAP 312 to AP 314 through the back-haul connection between the APs. Anumerically controlled oscillator using a stable temperature compensatedcrystal can use this pulse train to reproduce the clock frequency of AP.Then, AP 314 broadcasts an NDPA, followed by an NDP.

FIG. 12 illustrates flowchart 360 for a method of channel sounding whilecompensating for frequency offset performed by an AP. Initially, in step362, the AP receives a first packet from another AP. For example, thefirst packet is an NDPA containing oscillator frequency information inthe preamble. In another example, the first packet is a part of a pulsetrain.

Then, in step 364, the AP receives a second packet from the same AP. Inone example, the second packet is an NDP containing oscillator frequencyinformation in the preamble. In another example, the second packet is apart of a pulse train.

Next, in step 366, the AP calculates the OFO.

After calculating the OFO, the AP then transmits an NDPA in step 368.The NDPA may contain oscillator frequency information about this AP inthe preamble.

Finally, in step 370, the AP transmits an NDP. In an example, thepreamble of the NDP also contains oscillator frequency information.

FIG. 13 illustrates a graph of signal to noise ratio (SNR) in decibels(dBs) versus bit error rate (BER) for joint multi-user beamforming (JMB)with oscillator frequency error and with perfect synchronization. Curve302 shows the BER with oscillator frequency error, and curve 304 showsthe BER with perfect synchronization. Especially at a higher SNR, thejoint multi-user beamforming performs significantly better with perfectsynchronization than with an oscillator frequency error.

Orthogonal frequency division multiplexing (OFDM) performs well when theorthogonality of symbols is maintained. However, orthogonality ofsymbols may be reduced by inter-symbol interference (ISI) orinter-carrier interference (ICI). One source of distortion is carrierfrequency offset (CFO).

CFO may lead to phase noise due to the instability of the oscillatorsused at the transmitter and receiver. For example:

y[n] = IFFT{Y[k]} = IFFT{H[k]X[k] + X[k]}, and${y\lbrack n\rbrack} = {{\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{{H\lbrack k\rbrack}{X\lbrack k\rbrack}^{{{j2\pi}{({k + ɛ})}}{n/N}}}}} + {{Z\lbrack n\rbrack}.}}$

Thus, the inverse fast Fourier transform (IFFT) of

X[k]e^(j2πεn/N)

is calculated before passing the fading channel. The generated phaseoffset is ε, such that (−π≦ε<π)/2π. That is, |ε|≦0.5.

FIG. 14 illustrates a graph of SNR in dB versus BER with oscillatorfrequency error and without oscillator frequency error. Curve 332 showsthe BER with oscillator frequency error, and curve 334 shows the BERwithout oscillator frequency error. The BER is slightly lower with anoscillator frequency error.

The CFO is:

e^(j2π(ω) ¹ ^(−θ)+e) ^(j2π(ω) ⁰ ^(−θ).)

The CFO provides a cyclic delay diversity (CDD). The CFO is a lessimportant factor in performance degradation.

Synchronization may be performed using STFs and LTFs. Packet detectionmay be determined after observing about seven or eight peaks of thecorrelation of STF sequences. Also, the CFO and symbol timing offset maybe estimated in STFs. Then, the fine CFO may be determined using thenext two consecutive LTFs.

FIG. 15 illustrates timing diagram 340 for synchronization of APs usingSTFs and LTFs. Initially, the signal is detected, diversity is selected,and automatic gain control (AGC) is performed. Then, the coarsefrequency offset is estimated, and timing is synchronized. Next, thechannel and fine frequency offset are estimated. The rate length isdetermined, and data is transmitted.

Packets may be detected, for example, using autocorrelation orcross-correlation. For example,

${{R(d)} = {\sum\limits_{m = 0}^{L - 1}\left( {r_{d + m}^{*}r_{d + m + L}} \right)}},$

where r_(d) represents the value of the dth incoming sample forautocorrelation, or the dth STF sample for cross-correlation. Thenormalized value, M(d), of the correlation may be compared with acertain threshold. A match is found when the normalized value is above athreshold. For example,

${{M(d)} = \frac{{{R(d)}}^{2}}{\left( {P(d)} \right)^{2}}},$

where P(d) is:

$\sum\limits_{m = 0}^{L - 1}{{r_{d + m + L}}^{2}.}$

Also, {circumflex over (ε)}=ε₁+ε₂.

Once the packet detection is performed, after seven or eight peaks areobserved, the correlation continues, and two or three more peaks arefound. R(d) is found, where d is the peak position. An angle of R(d) atthe peak positions repeats for two consecutive peaks. The average angleof the two or three peak positions is the coarse CFO estimation of−2πε₁.

After the coarse CFO estimation is performed, the correlation continues.The window size for the correlation is 64, because the FFT size of theLTFs is 64. The procedure is performed similarly, with the coarse CFOestimation of −2πε₂.

The final CFO estimation is −4π(ε₁+ε₂). FIG. 16 illustrates a graph ofSNR in dB versus mean squared error (MSE) on coarse and fine CFOestimation. Curve 352 shows the MSE as a function of SNR.

FIG. 17 illustrates a block diagram of processing system 270 that may beused for implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system may comprise a processing unitequipped with one or more input devices, such as a microphone, mouse,touchscreen, keypad, keyboard, and the like. Also, processing system 270may be equipped with one or more output devices, such as a speaker, aprinter, a display, and the like. The processing unit may includecentral processing unit (CPU) 274, memory 276, mass storage device 278,video adapter 280, and I/O interface 288 connected to a bus.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. CPU 274 may comprise any type of electronic dataprocessor. Memory 276 may comprise any type of system memory such asstatic random access memory (SRAM), dynamic random access memory (DRAM),synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof,or the like. In an embodiment, the memory may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms.

Mass storage device 278 may comprise any type of storage deviceconfigured to store data, programs, and other information and to makethe data, programs, and other information accessible via the bus. Massstorage device 278 may comprise, for example, one or more of a solidstate drive, hard disk drive, a magnetic disk drive, an optical diskdrive, or the like.

Video adaptor 280 and I/O interface 288 provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface card (not pictured) may be used to provide a serialinterface for a printer.

The processing unit also includes one or more network interface 284,which may comprise wired links, such as an Ethernet cable or the like,and/or wireless links to access nodes or different networks. Networkinterface 284 allows the processing unit to communicate with remoteunits via the networks. For example, the network interface may providewireless communication via one or more transmitters/transmit antennasand one or more receivers/receive antennas. In an embodiment, theprocessing unit is coupled to a local-area network or a wide-areanetwork for data processing and communications with remote devices, suchas other processing units, the Internet, remote storage facilities, orthe like.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A collaborative service set (CSS) comprising: acontroller access point (AP) configured to be associated with a firstplurality of stations and a first member AP, wherein the first member APis associated with a second plurality of stations, wherein thecontroller AP is configured to coordinate transmissions between thefirst member AP and the second plurality of stations with transmissionsbetween the controller AP and the first plurality of stations, whereinthe controller AP and the first member AP are configured to transmitmessages simultaneously.
 2. The CSS of claim 1, wherein a first stationof the first plurality of stations is configured to freely associatewith the first member AP.
 3. The CSS of claim 1, wherein the firstmember AP is a leader AP.
 4. The CSS of claim 1, wherein the controllerAP is a leader AP.
 5. The CSS of claim 1, wherein the controller AP isconfigured to be associated with a plurality of member APs comprisingthe first member AP.
 6. The CSS of claim 1, wherein the controller AP,the first member AP, and the first plurality of stations are coupled bya signaling channel.
 7. The CSS of claim 1, wherein the first pluralityof stations are associated with a plurality of unique stationidentification numbers (IDs) and a plurality of media access control(MAC) addresses, wherein a length of each of the plurality of uniquestation IDs is shorter than a length of each of the plurality of MACaddresses.
 8. The CSS of claim 7, wherein the plurality of uniquestation IDs is a plurality of association identification numbers (AIDs).9. The CSS of claim 1, wherein the controller AP and the first member APare coupled by a distribution system.
 10. A method of channel soundingin a collaborative service set (CSS), the method comprising:broadcasting, by a first access point (AP), a first null data packetannouncement (NDPA) comprising a short identification number (ID) of afirst station; broadcasting, by the first AP, a first null data packet(NDP) after transmitting the first NDPA; receiving, by the first AP froma second AP, a second NDPA; receiving, by the first AP from the secondAP, a second NDP after receiving the second NDPA; transmitting, by thefirst AP to the first station, a first polling frame after transmittingthe first NDP and receiving the second NDP; and receiving, by the firstAP from the first station, a first feedback report after transmittingthe first polling frame.
 11. The method of claim 10, wherein the firstAP is a leader AP, and wherein receiving the second NDPA is performedafter transmitting the first NDP.
 12. The method of claim 10, whereinthe second AP is a leader AP, and wherein transmitting the first NDPA isperformed after receiving the second NDP.
 13. The method of claim 10,further comprising: receiving, by the first AP from a second station, asecond polling frame; and receiving, by the first AP from the secondstation, a second feedback report after receiving the second pollingframe.
 14. A method of channel sounding in a collaborative service set(CSS), the method comprising: receiving, by a station from a firstaccess point (AP), a first null data packet announcement (NDPA)comprising a first simplified short identification number (ID) of thestation; receiving, by the station from the first AP, a first null datapacket (NDP) after receiving the first NDPA; receiving, by the stationfrom a second AP, a second NDPA after receiving the first NDP;receiving, by the station from the second AP, a second NDP afterreceiving the second NDPA; estimating a first channel between thestation and the first AP in accordance with the first NDP to produce afirst estimated channel; receiving a first polling frame, by the stationfrom the first AP; and broadcasting, by the station, a first feedbackreport in accordance with the first estimated channel after receivingthe first polling frame.
 15. The method of claim 14, further comprising:estimating a second channel between the station and the second AP inaccordance with the second NDP to produce a second estimated channel;receiving a second polling frame, by the station from the second AP; andbroadcasting, by the station, a second feedback report in accordancewith the second estimated channel.
 16. A method of determining anoscillator frequency offset between a first access point (AP) and asecond AP, the method comprising: receiving, by the second AP from thefirst AP, a first packet comprising a first preamble; receiving, by thesecond AP from the first AP, a second packet comprising a secondpreamble after receiving the first packet; and determining an oscillatorfrequency offset between the first AP and the second AP in accordancewith the first preamble and the second preamble.
 17. The method of claim16, wherein the first packet is a first null data packet announcement(NDPA) and the second packet is a first null data packet (NDP).
 18. Themethod of claim 16, further comprising: broadcasting, by the second AP,a second NDPA after calculating the oscillator frequency offset; andbroadcasting, by the second AP, a second NDP.
 19. The method of claim18, further comprising waiting a first processing time receiving thefirst packet before broadcasting the second NDPA.
 20. The method ofclaim 16, wherein determining the oscillator frequency offset comprises:estimating a coarse carrier frequency offset; and estimating a finecarrier frequency offset.
 21. The method of claim 16, wherein the firstpreamble comprises a short training field and a long training field. 22.The method of claim 16, wherein a pulse train comprises the first packetand the second packet, and wherein determining the oscillator frequencyoffset comprises reproducing a clock frequency of the first AP by astable temperature compensated crystal in accordance with the pulsetrain.